This relates to video signals and, more particularly, to a system and a method for employing video signals to store and retrieve digital information; with the storage medium being transitory, such as a transmission cable, or permanent, such as video discs.
Generally, analog video signals are transmitted and stored with the same format. The storing medium most often used is magnetic tape, but the storage of both still and moving pictures on video discs has been a long-standing goal. The present commercially available formats are largely the result of work begun by Philips and RCA in the middle 1960's. Although many disc storage approaches exist for special purpose applications, only three presently seek mass market acceptance. These are the RCA CED (Capacitance Electronic Disc) Selecta Vision system, the Philips Laser Vision system, used also by Pioneer and others, and the VHD (Video High Density) system developed by the Japan Victor Corp., used also by GE. The basic approach of all video disc systems is the same as that of the phonograph record; i.e., information is recorded on the disc in a spiral track and reproduced by a read-head which moves radially across the spinning disc. The various systems differ as to how the information is stored on the disc surface and what method is used to recover it.
The Philips system, for example, uses a disc which is a plastic "sandwich" consisting of transparent covering over the information bearing base. The base has a very thin aluminized coating whose reflectance variations provide the desired output signal. Two such structures are glued back-to-back to form a two-sided disc. The original master is made by writing a pattern onto a special surface with a high-power laser. High-volume duplicates are made from the master by stamping techniques not unlike those used for reproducing LP phonograph records. The disc rotates at 1800 rpm during playback and must be manually turned over to play both sides.
On each disc, information is stored in a spiral track of pits which are constant in width and depth (400 nm and 110 nm, respectively) but variable in length and spacing (average length=640 nm). On the standard 30 cm disc there are 54,000 turns to the spiral, which provide 30 minutes of continuous program material. Normal play is from inside out, thus allowing a common starting point for discs of varying diameter.
During recording a standard NTSC video signal frequency modulates a carrier with a modulation index such that 7.6 MHz corresponds to sync tip, 8.1 MHz to blanking and 9.3 MHz to peak white. Simultaneously, two wide band (20 KHz) audio signals frequency modulate carriers of 2.3 and 2.81 MHz. The three signals are combined (viz., the modulated audio waves are impressed onto the clipped FM video signal by symmetrical double edge pulse width modulation) and the composite signal then drives the recording laser.
During playback, the signal read from the disc is filtered to separate audio and video components which are individually demodulated. The 30 rps disc rotation means that exactly one video frame is recorded per revolution. This facilitates still frame playback, which is accomplished by causing the read beam to jump back one track at the end of a frame.
Video discs have been used extensively for the purpose for which they were designed, to wit, recording and playing video information. Recently, however, there has been a rapidly growing interest in using video disc technology as a new, dynamically interactive way of presenting material now contained in books, encyclopedias, magazines, catalogs, instruction manuals, etc. Video discs offer a unique set of characteristics for this purpose. They have a very high information storage density (roughly 50 times that of magnetic materials), rapid random access of addressable information, low cost mass produced copies, and relatively low cost playback equipment. By interfacing a video disc player and associated TV display to a computer, stored audio/visual data can be accessed interactively in a variety of user-friendly ways, such as in video games or menu-driven retrieval and/or control processes.
Heretofore, even in applications of "information" storage, video information has been stored in an analog fashion using NTSC or PAL formats. The audio signals (stereo) in those systems are frequency modulated and added to the modulated video signal. More recently, some work has been done on digital audio modulation. For example, K. A. S. Immink et al. reported at the 74th Convention of the Audio Engineering Society (October 1983) on a system for recording digital audio with the LaserVision analog video system. In their system, the digital audio is converted to the Compact Disc Digital Audio System format (CD format). Then it is passed through a low pass filter (having a cutoff frequency of approximately 1.75 MHz), pre-emphasized, and applied to the pulse width modulation input of the video modulator.
This dual format has a number of disadvantages and limitations. The most serious is the interference between the audio and video spectra and the loss of high frequency detail in the video spectrum. This occurs because, in order to accommodate the spectrum of the digital audio the video spectrum must be band limited. In addition, there is the complexity involved in recording both formats simultaneously. It may be noted that the original application of recording in both formats on a video disc was indeed to provide high quality (CD) audio at the expense of the video image.
Despite the great potential of interactive video disc players, commercially available versions have important limitations. In particular, audio accompaniment to the video display is possible only in the standard thirty frame/second playback mode and not with other modes such as slow motion or still frame. Also, the frequent employment of the still frame mode and the close viewing distance common to interactive terminal use both serve to magnify the imperfections of the universally used NTSC video format.
Other attempts of analog recording of digital information within video include methods to encode digital bit patterns in an analog muti-level NRZ format. The specific formats employed are proprietary but, to the extent known, they are nonstandard and, in general, they lack error correction, recovery and control, immunity from burst errors and are characterized by a poor signal-to-noise ratio due to the use of many analog levels.
In view of these problems and other considerations, it is an object of my invention to employ a robust method for encoding digital information onto video signals.
It is another object of my invention to encode digital information within a standard video signal format.
It is still another object of our invention to employ a recording format that requires a minimum of hardware, permitting the use of a robust recording formula without an attendant high cost of playback equipment.